Hybrid cars and the like are provided with a booster circuit in their system for supplying electric power to the motor. The booster circuit includes a reactor as a component. The reactor has a structure in which a coil is wound around a core. When such a reactor is used in an alternating magnetic field, the core produces an energy loss known as an iron loss. Generally, the iron loss is expressed as the summation of a hysteresis loss and an eddy current loss and it becomes noticeable in the use at high frequency, in particular.
To decrease the above-described iron loss, the core of the reactor is sometimes formed by using a dust core. To form a dust core, first, a soft magnetic material is prepared that is composed of composite magnetic particles composed of soft magnetic metallic particles each coated with an insulating film. Then, the soft magnetic material is pressed to form the dust core. Because the metallic particles are insulated with one another with the insulating film, the dust core is highly effective in decreasing the eddy current loss, in particular.
Despite the above description, because the dust core is produced through the press molding, the pressure at the time of the press molding may damage the insulating films of the composite magnetic particles. When the insulating films are damaged, the soft magnetic metallic particles in the dust core are brought into contact with one another. This contact causes the eddy current loss to increase and thus may decrease the high-frequency property of the dust core.
In addition, strain and dislocation introduced into the soft magnetic metallic particles during the press molding cause an increase in the hysteresis loss. To prevent this increase, it is necessary to perform heat treatment after the press molding. The heat treatment, however, may deteriorate the insulating film, so that it is undesirable to perform the heat treatment at high temperature. When the heat treatment temperature is not sufficiently high, the strain and the like introduced into the metallic particles cannot be removed sufficiently. As a result, the hysteresis loss may increase, thereby decreasing the high-frequency property of the dust core.
To solve the problem caused by the press molding and heat treatment, a technique, for example, described in Patent Literature 1 forms on the surface of each of the soft magnetic metallic particles an insulating layer having multiple layers composed of an insulating film, a heat-resistance-imparting protective film, and a flexible protective film. According to the technique described in this literature, the insulating film may be formed of a phosphorus compound, a silicon compound, or the like, the heat-resistance-imparting protective film may be formed of an organic-silicon compound or the like, and the flexible protective film may be formed of silicone or the like.